1. Field of the Invention
This invention relates to supercritical fluid chromatography or separations and column packing material therefor. More particularly, this invention relates to alumina packing material or a packing material comprising a metal oxide/hydroxide reacted with a phosphorous-containing acid, e.g., phosphonic or phosphinic acid, or phosphoric ester to form preferably a monomolecular layer thereon thereby providing a supercritical fluid chromatographic packing material having good pH stability as well as high efficiency.
2. Description of the Related Art
Chromatographic packing materials made from organic resins suffer from poor physical strength, poor thermal stability, high cost, solvent swelling, and low capacity. Chromatographic packing materials made from metal oxides such as silica exhibit poor chemical stability at high pH. For many applications, a chromatographic packing material which high physical integrity, good chemical stability over high and low pH conditions, specific surface functionalities, good thermal stability, and low cost is needed.
Other metal oxides such as alumina have been used as adsorbents because of the good physical integrity and low cost of alumina. The solubility of alumina in pH ranges between 4 and 9 is very low and the material is, therefore, chemically and physically stable in this pH range. However, beyond this pH range, on either the basic or acidic side, alumina becomes soluble in aqueous media and its physical strength and integrity degrades rapidly.
Modifications of metal oxide adsorbents such as alumina and aluminosilicates have been proposed. Stockel U.S. Pat. No. 4,506,628 teaches the formation of an adsorbent animal litter utilizing alumina, aluminosilicates, or coal residues as the substrate intimately mixed with monomers containing acid functionalities which polymerize in situ. The monomer, such as vinyl phosphonic acid, together with a redox catalyst, is mixed with a pliable dough formed from alumina and water and extruded into pellets which harden as the monomer polymerizes.
Modified alumina has also been used in the formation of catalysts. Johnson et al U.S. Pat. Nos. 4,202,798 and 4,251,350 describe the formation of a hydrocarbon hydrotreating catalyst formed by contacting alumina with a phosphorous-containing acid compound such as phenylphosphonic acid and then calcining the phosphorous-containing hydrous alumina. The calcined alumina is then treated with at least one metal-containing compound and again calcined to form the catalyst product.
In addition, Cupery U.S. Pat. No. 3,013,904 discloses a substrate having an organic polymer containing pentavalent phosphorus bonded thereto. Coatings of phosphorous-containing organic polymers are applied over coatings of positively charged colloidal metal oxides applied to negatively charged substrates. The thickness of the combined colloidal oxide and polymer layers on a substrate is less than 100 millimicrons.
Novotny et al U.S. Pat. No. 4,479,380 discloses an apparatus open-tube supercritical fluid chromatography wherein the apparatus has an elongated passageway having inlet and outlet ends such as a capillary column, coated or not with a coating having affinity for solute molecules to be analyzed. Coating materials include methyl and phenyl silicine polymers and adsorbents such as silica, alumina and activated carbon.
Venables et al U.S. Pat. No. 4,308,079 teaches the treatment of an aluminum oxide surface of an aluminum substrate with a monomolecular layer of an amino phosphonate compound such as nitrilotris (methylene) triphosphonic acid to retard hydration of the aluminum oxide to aluminum hydroxide to provide a more stable microporous surface which is particularly suited to commercial adhesives. The presence of the hydrated oxide is said to interfere with the formation of a satisfactory bond between the adhesive and the oxide, while the phosphonate treatment is said to inhibit the conversion of the oxide to hydroxide without interfering with subsequent bonding of the adhesive to the oxide.